The long-term objective of this research is to provide clinicians and researchers with better instruments to measure clinically-relevant health-related outcomes for consumers of orthotic and prosthetic services. Using modern measurement theory, a dynamic measurement system will be developed to provide valid, reliable, and interpretable measures that can be readily integrated into existing health care and research systems. Two specific aims are proposed to develop an outcome system specific to individuals with lower limb amputation (LLA) as well as to substantially expand our understanding of the impact of prosthetic devices on health and quality of life. Aim 1 proposes to identify, analyze and improve existing instruments to measure clinically-relevant outcomes in individuals with LLA and to develop a new item bank, short forms, and computer adaptive tests for measuring mobility using modern psychometric methods. To identify relevant domains affected by prosthetic use, existing measures of pain impact, depression, fatigue, and physical function and mobility will be administered in a national, cross-sectional study of individuals representing those with the most prevalent levels and etiologies of amputation. Structural equation modeling and other advanced latent variable modeling methods will be used to develop a model of most relevant outcomes and quantify their contribution to patient health-related quality of life. Cross-sectional data will be used to examine psychometric functioning of the instruments in people with LLA and to conduct psychometric analyses of the new mobility items. The associations among outcomes and client-specific variables (i.e., those related to medical history and prosthetic prescription) will be used to develop profiles that will allow researchers to evaluate and compare the effectiveness of therapeutic and prosthetic interventions. Aim 2 proposes to conduct a longitudinal validation study of the newly developed item bank by following individuals with LLA receiving prosthetic care over a period of 18 months. The criterion validity of the new item bank, the sensitivity to change (i.e., provision of new or replacement prosthetic limb) of the mobility score, and their associations with other measures and specific health markers will be used to validate the outcomes system. The proposed study will develop a scientifically-based, validated measurement system to evaluate health-related outcomes in individuals with LLA. The system will be used to examine longitudinal trajectories of pain impact, depression, fatigue, physical function and mobility and the impact of prosthetic interventions on these outcomes. This will facilitate research, inform treatment strategies, and improve patient care for individuals with LLA. This project has the potential to aid in the selection of clinical interventions, facilitate prosthetics research, inform reimbursement policies, improve the quality of clinical practice, and enhance outcomes for individuals with limb loss.